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Data are drawn from the National Longitudinal Survey of Youth (NLSY), which focuses on the labor market experiences of adolescents and has followed a representative sample of 12,686 youth ages 14 through 21 years for 23 years, from their adolescence into their early 40's. The NLSY includes a battery of questions about drug use that were administered during 5 years of the survey, in 1984, 1988, 1992, 1994, and 1998. Data were analyzed using logistic regression analysis and odds ratios. Results revealed that most respondents reported no drug use during the 5 years under examination. Marijuana use was reported by 42 percent of respondents, while 19 percent reported cocaine use and 3 percent reported crack cocaine use. Most respondents only reported use during one survey year or used the drug for only one more survey year. However, half of marijuana users who used in one survey year reported marijuana use in the next survey year. Approximately one-quarter of cocaine users reported cocaine use in the next survey year. In terms of predictors of heavy drug use as adults, results indicated that heavy marijuana use in adolescence was predictive of heavy cocaine use in adulthood. Results also showed that young male drug users were nearly twice as likely as young female drug users to become heavy adult drug users. Adolescents reporting significant amounts of illegal income were also twice as likely to become heavy cocaine users. The findings suggest that steering adolescents away from heavy marijuana use and criminal occupations may be an effective means of deterring future heavy drug use. Footnotes, exhibits, appendix.